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Originally Posted by RobBeck
The ID of the TD04 can is ~2.000".
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So then the diaphragm area is approximately pi square inches. If 6psi is full open, 6psi * 3.141592653589793238in^2 (that was off the top of my head, btw
) = almost 19lbs. So if the wastegate port is 20mm diameter, that's about 0.5sq in area, and it would take 38psi of exhaust pressure to force it open.
Now, we're talking fairly vague figures here. If the wastegate actuator cracks the flapper off its seat at 6psi, those numbers are accurate (I've simply solved backwards to find for the force holding it on its seat). AS SOON AS the flapper lifts from the seat, exhaust pressure is reduced somewhat, and continues to be reduced as the flapper is opened further.
With a 23mm port, that number is reduced to 29.5psi exhaust pressure.
With a 27mm port, that number is further reduced to 21.5psi exhaust pressure.
Why go bigger? Personally, I'd recommend a 10psi actuator and a stock wastegate port to just about anybody, and higher than that would still be quite reasonable.
Again, the biggest reason dual-port externals work so well is because of the large diphragm and pressure exerted to keep the valve closed. How effective they are at negating the effects of high exhaust backpressure are related to the ratio of the areas (diaphragm to exhaust valve) and the ratio of intake boost pressure to exhaust turbine backpressure. Of course, this effect is DOUBLED if you mount the wastegate so that exhaust pressure forces it shut instead of open, in which case you could have a creep problem without good boost control logic.
-Chris